A method for spray application of a monolithic refractory material can be roughly classified into a wet application method and a dry application method.
In the wet application method, a monolithic refractory material slurried by adding application water to a raw material and kneading (or mixing) them is pressure-fed (i.e., fed under pressure), and sprayed together with an accelerator (i.e., set accelerating agent) or the like added thereto in a distal nozzle section. In the dry application method, a monolithic refractory material is air-carried (i.e., fed in an air-carried manner) in a dry state, and sprayed together with application water injected thereinto in a distal nozzle section.
As compared with the dry application method, the wet application method has advantages, for example, of being able to form a dense refractory spray-applied product excellent in adhesion and to reduce an amount of dust generation during the application. At the same time, the wet application method has disadvantages of the need for being equipped with a kneading device and a slurry pressure-feeding device which are structurally complicated and costly, and a time-consuming post-spraying operation of cleaning the slurried monolithic refractory material attached on the kneading device and an inner surface of a feed hose.
In this respect, the dry application method having a simple scheme of injecting application water into an air-carried dry monolithic refractory material in a distal nozzle section can have a simplified spray application apparatus and higher operation efficiency. However, the monolithic refractory material, i.e., a spray material, is sprayed without allowing the water to be sufficiently mixed therewith. Thus, a large amount of dust (coarse particulate) is generated during the spraying, and a refractory structure of a spray-applied product is apt to become nonuniform, to cause deterioration in deposit efficiency, bonding strength and corrosion resistance. Moreover, the dry application method has a disadvantage of difficulty in obtaining a dense spray-applied product due to lower kneading (or mixing) effect and the use of a larger amount of water as compared with the wet application method.
As an improvement in the conventional dry application method, there has been previously studied a dry application technique of injecting water into a spray-material feed passage of a hose for feeding a spray material to aim for a kneading effect based on an internal collision of the spray material or a collision between the spray material and an inner surface of the hose. This technique has an advantage of suppressing the dust generation which is one disadvantage in the conventional dry application method, and improving the quality of a spray-applied product.
For example, the following Patent Publication 1 discloses a spray application method which comprises adding water at two positions of a line of spray-material-feeding compressed air and a spray nozzle. The Patent Publication 1 includes a description about injecting water into the spray-material-feeding compressed air as a preliminary kneading. Specifically, it is described that the water added into the air in an evenly dispersed manner makes it possible to solve a problem about insufficient kneading so as to allow the use of a shorter nozzle and prevent an attachment of a spray material on an inner surface of a hose due to locally uneven dispersion of water. While this method would be really effective in reducing an amount of dust generation and suppressing an attachment of the spray material on the inner surface of the hose as compared with the case of adding water only at the spray nozzle, an adequate kneading effect cannot be obtained because the spray material is hardly evenly wetted with water if it is simply sprayed. Therefore, this technique could not provide a denser spray-applied product with lower water content as compared with the conventional dry application method.
The following Patent Publication 2 discloses a two-stage water injection-type spray application method which comprises adding application water in an amount capable of obtaining a castable working-consistency, into a air-carried monolithic refractory material together with compressed air through a watering device just before a spray nozzle, and additionally adding an coagulant or a shape-retentivity imparting agent into the monolithic refractory material at the spray nozzle. According to the description of the Patent Publication 2, in this method, a strong stirring effect derived from adding the application water using high-pressure compressed air allows the water to be evenly mixed with the monolithic refractory material within a short time of period. This makes it possible to solve a problem in conventional dry and semi-wet spray application methods, such as deterioration in working environment due to a rebound loss and deterioration in quality of a spray-applied product due to an increase in water content, and other problems, such as structural complexity of a spray application apparatus, a time-consuming cleaning operation and a large disposal amount of residual spray material. However, in this spray application method, the watering device is designed to add the water in a large amount allowing the spray material to be slurried at once and add the water just before the spray nozzle. This makes it difficult to evenly disperse the water in the spray material and obtain a sufficient kneading effect. Therefore, this spray application method has a problem of difficulty in obtaining a dense spray-applied product. While it is contemplated to arrange the watering device at a position distant from the spray nozzle so as to improve the kneading effect, a feed hose will be likely to cause clogging and a time for the cleaning operation will be increased.
The following Patent Publication 3 discloses a spray application method which comprises feeding a spray material including a refractory aggregate, a refractory powder, a binder, a dispersant and an accelerator, on an airstream into a feed pipe, injecting the entire amount of application water into the spray material at an intermediate position of the feed pipe, further airs-feeding (air-carrying) the spray material, and spraying the spray material through a spray nozzle. According to the description of the Patent Publication 3, even after the entire amount of application water is injected at the intermediate position of the feed pipe to wet the spray material, the monolithic refractory composition is not changed to have a viscosity causing attachment on the feed pipe, and the water to be added can be limited to an amount approximately equal to that in a conventional wet application method to obtain a high-quality spray-applied product.
However, in an actual test based on this method, it was found that, when the entire amount of application water is simply injected at an intermediate position of a feed pipe, a kneaded state becomes insufficient due to a poor affinity of water for the refractory powder, particularly, ultrafine powder materials therein, to cause an increase in rebound loss and an amount of dust generation and deterioration in workability (i.e., decrease in working-consistency), which leads to an disadvantage of hardly obtaining a spray-applied product with stable quality.
Thus, an amount of application water is inevitably increased to ensure an adequate workability. This causes a problem about a local variation in water content of a spray-applied product. Specifically, a region with an excessively large water content of a spray-applied product has a high porosity, and conversely a region with an excessively small water content of a spray-applied product has an insufficient binding. As the result, a desired durability cannot be obtained in both the regions. Moreover, if the application water is added at a position distant from a nozzle hole, the spray material starts aggregating due to melt of the accelerator, to causes problems about the need for increasing an amount of application water, and an increase in time for the bothersome operation of cleaning the feed pipe.
The Patent Publication 3 also includes a description about adding water as a pre-moisture. This water is added to aim for a dust-generation protection effect, and therefore a preliminary kneading effect cannot be adequately obtained by adding such an amount of water.                [Patent Publication 1] Japanese Patent Laid-Open Publication No. 63-31562        [Patent Publication 2] Japanese Patent Laid-Open Publication No. 10-316478        [Patent Publication 3] Japanese Patent Laid-Open Publication No. 2002-220288        